This invention relates to an optical glass and, more particularly, to an optical glass having a low glass transition temperature (Tg) and high refractive index and low dispersion characteristics, being light and suitable for precision press molding.
There are spherical lenses and aspherical lenses as lenses used for constituting an optical system. Many spherical lenses are produced by lapping and polishing glass pressings obtained by reheat press molding glass materials. On the other hand, aspherical lenses are mainly produced by precision press molding, i.e., the method according to which lens preforms which have been softened by heating are press molded with a mold having a high precision molding surface and the shape of the high precision molding surface of the mold is transferred to the lens preforms.
In obtaining glass moldings such as aspherical lenses by precision press molding, it is necessary to press lens preforms which have been softened by heating in a high temperature environment for transferring the shape of the high precision molding surface of the mold to the lens preforms and, therefore, the mold used for such precision press molding is subjected to a high temperature and, moreover, a high pressing force is applied to the mold. Hence, in heating and softening the lens preforms and press molding the lens preforms, the molding surface of the mold tends to be oxidized or eroded, or a release film provided on the molding surface tends to be damaged with the result that the high precision molding surface of the mold cannot be maintained or the mold itself tends to be damaged. In such a case, the mold must be replaced and, as a result, frequency of replacement of the mold increases and production of products at a low cost in a large scale thereby cannot be achieved. Accordingly, glasses used for precision press molding are desired to have the lowest possible glass transition temperature (Tg) from the standpoint of preventing such damage to the mold, maintaining the high precision molding surface of the mold for a long period of time and enabling precision press molding at a low pressing force.
In conducting precision press molding, the glass of a lens preform needs to have a mirror surface or a surface close to a mirror surface. A lens preform generally is either produced directly from molten glass by the dripping method or produced by lapping and polishing glass pieces. In case of producing a lens preform by lapping and polishing, since the surface portion of a glass is scraped off, piece and the glass piece can be used without any particular problem even if there is a defect in appearance such as devitrification which occurs in the surface portion only of the glass piece. Such lens preform made by lapping and polishing includes, for example, a lens preform made by lapping and polishing the surface of a reheat press product, and a lens preform processed to a spherical shape having very little deviation from true sphere.
An optical system which uses such optical glass is mounted on an optical product such as digital cameras. Conforming to recent tendency towards compact and light design, there is a demand for a light lens also in lenses constituting an optical system.
For these reasons, from the point of view of utility for optical design, there has been a strong demand for an optical glass having high refractive index and low dispersion characteristics, a low glass transition temperature (Tg) and a light weight.
There has particularly been a strong demand for a high refractive index and low dispersion optical glass having optical constants of refractive index (nd) within a range from 1.73 to less than 1.80 and Abbe number (νd) within a range from 43 to 55.
Since a high refractive index and low dispersion optical glass is very useful in the optical design, various glasses of this type have for a long time been proposed.
Japanese Patent Application Laid-open Publications No. 2002-249337 discloses an optical glass having refractive index within a range from 1.72 to 1.83 and Abbe number within a range from 45 to 55. The optical glass disclosed in this publication, however, comprises La2O and Gd2O3 as main components and does not comprise Y2O3 and, therefore, it has large specific gravity with resulting difficulty in realizing a compact and light design. Further, optical glasses described as examples of this publication have excessively high glass transition temperatures and, therefore, these optical glasses are not suitable for precision press molding.
Japanese Patent Application Laid-open Publication No. 2003-201143 discloses an optical glass having refractive index within a range from 1.75 to 1.85 and Abbe number within a range from 40 to 55. The optical glass disclosed in this publication, however, comprises La2O and Gd2O3 as main components and does not comprise Y2O3 and, therefore, it has large specific gravity with resulting difficulty in realizing a compact and light design.
Further, in the optical glasses specifically disclosed in these publications which satisfy the above described optical constants, the ratio of SiO2/B2O3 in weight % is outside of the range from 0.30 to 1.55 and the ratio of Y2O3/La2O3 in weight % is outside of the range from 0.15 to 1.00 and, for this reason, fail to satisfy desirable properties of the optical glass of the present invention which will be described below.
It is, therefore, an object of the present invention to provide an optical glass which has comprehensively eliminated the above described drawbacks of the prior art optical glasses and has the above described optical constants, a low glass transition temperature (Tg) and a light weight and therefore is suitable for precision press molding.